Plasma coatings, and low pressure plasma coatings in particular, are today widely used to add functionalities to substrates, such as hydrophilic, hydrophobic, oleophobic, scratch resistant properties and/or barrier coatings. On some substrates, especially dark substrates, such as black, grey, dark blue, dark green, dark purple substrates, but also substrates having a high gloss surface or a low surface roughness (e.g. soft feel surfaces or polished surfaces), it is an unwanted phenomenon that these substrates tend to become darker in colour after treatment. Sometimes a rainbow-like discolouration becomes visible.
In those situations where the substrate is a material or object that is as such used by the end customer, it is unwanted that this colour change, whether it is a darkening of the colour or the appearance of a rainbow effect, becomes visible to the end user.
In order to solve this issue, applicant has invented a so-called “plasma diffuser”, which allows to deposit nanocoatings with highly reduced to none discolouration effect while at the same time the performance of the substrate is not negatively influenced.
This invention allows to treat products at the end of the manufacture line as well as during manufacture.
Applicant noticed that the effect of discolouration is the strongest for halogen containing coatings, such as fluoro-containing coatings used to impart hydrophobic and/or oleophobic properties to substrates.
The effect is the most pronounced on black, dark blue, dark green and dark grey surfaces. The effect is pronounced on high gloss, high reflective surfaces as well, and also on surfaces which have a rather smooth surface on macroscopic level, like flat plastic parts, such as the casing of (hand held) electronic devices, or sheets or garments made of textile.
Applicant noticed that discolouration is more pronounced for batch processes, during which substrates are coated in a static way. In static processes, sheets of textiles and garments are for instance fixed in a vertical position, or plastic parts and electronics can be coated in a tray-wise system with horizontal position.
In the beginning of the development of plasma processing, the used systems were limited in volume, for example less than 10 litres, even less than 5 litres or less than 1 litre. Such small systems have been designed mainly for use in the semiconductor industry, with a good plasma uniformity and density distribution because of the limited size of the system, as well as well controllable process parameters. The design of the systems was optimized and often complex for treating smaller and/or limited amounts of products which high precision. Discolouration was not an issue in this type of equipment since products to be treated are small and so is the system.
Yasuda, H. and Hsu, T. describe the use of a cylindrical glass chamber of 4 mm diameter and 10 cm length, representing a chamber volume of approx. 1.26 cm3, or 0.00126 l (“Some Aspects of Plasma Polymerization Investigated by Pulsed R.F. Discharge”, Journal of Polymer Science: Polymer Chemistry Edition, vol. 15, 81-97 (1977); and “Some Aspects of Plasma Polymerization of Fluorine-Containing Organic Compounds”, Journal of Polymer Science: Polymer Chemistry Edition, vol. 15, 2411-2425 (1977)).
Panchalingam V. et al. describe the use of a cylindrical glass chamber of 10 cm diameter and 30.5 cm length, representing a chamber volume of approx. 2.4 litres (“Pulsed Plasma discharge Polymer Coatings”, ASAIO Journal, 1993, M305-M309).
Hynes, A. M et al. describe the use of a cylindrical glass reactor of 5 cm diameter and a volume of 490 cm3 (0.49 litres) (“Plasma Polymerization of trifluoromethyl-substituted Perfluorocyclohexane Monomers”, Macromolecules 1996, 29, 18-21, and “Pulsed Plasma Polymerization of Perfluorocyclohexane”, Macromolecules 1996, 29, 4220-4225).
U.S. Pat. No. 4,737,379 (Energy Conversion Devices Inc.) describes for example a small plasma equipment having a tubular chamber or vessel, used to deposit hard alloy coating free of hydrogen, a.o. for passivation purposes, on for example semiconductors.
U.S. Pat. No. 4,686,113 (Fairchild Semiconductor Corporation) describes a plasma equipment consisting of a quartz tube as chamber for inductively coating silicon substrates, e.g. used in semiconductor applications.
GB Patent application No. 2,220,006 (Philips Electronic Associated) describes the use of the plasma chamber for coating or etching substrates, e.g. semiconductor bodies. In the description it is mentioned that the electrodes are approx. 15 cm×15 cm and the samples treated in the examples are of a 100 mm diameter, indicating a small system for treating small objects.
Other examples of chambers used at reduced pressure are chambers for atomic layer deposition processes (ALD). These processes are very complex, and several prior art documents relate to the design of these ALD vacuum chambers in order to guarantee a good process. Typically these systems have a limited volume since larger systems lead to less controllable processing parameters and thus to less performant treatments.
For example, US Patent application No. 2009/255,470 (Beneq Oy) describes an ALD reactor for treating small objects with an ALD process, where the design is chosen in a way that gas may enter the reaction chamber through all walls to optimize the gas distribution. Page 3 indicates a chamber with an inner diameter of 230 mm, for treating one or more silicon samples having a diameter of 200 mm.
U.S. Pat. No. 4,389,973 (Oy Lohja AB) describes multiple complex designs for an ALD reactor in order to optimize the process and the resulting coating. The process uses gas phase diffusion barriers in order to separate the single reaction steps of the ALD process. Again, the system is developed for treating a limited number of samples having limited dimensions.
US Patent application No. 2010/166,955 (Cambridge NanoTech Inc.) describes an apparatus having one or more smaller rectangular reaction chambers vertically positioned on top of each other, wherein one substrate is placed in one reaction chamber at a time. Typical chambers have a volume of 20 litres, and are used to deposit thin films by ALD (atomic layer deposition) or ALE (atomic layer epitaxy) onto substrates for use in LCDs. The chamber design is optimized to obtain a substantially uniform flow direction and velocity. The design is complex and the dimensions of the substrates to be coated, as well as the throughput, is limited.
EP Patent application No. 1,933,608 (Tonen Chemical Corporation) describes a method and apparatus for plasma-treating a porous body by either blowing a plasma gas to the porous body or by sucking a plasma through the porous body or a combination of both. However, this does not guarantee a coating with a uniform spread as is desired. Especially when applying this technique to nonporous bodies, this will create an asymmetrical coating, but also when applied to porous bodies, as the delivery of the plasma gas will still differ from the side from which it is supplied to the opposing side of the porous body. Furthermore, the substrate is supported by a porous body on one side. While allowing some passage of the plasma gas, this will nonetheless to some degree restrict the access of the plasma gas to the supported side of the substrate because of the contact between the porous body and the substrate and create a non-uniform coating on said side.
U.S. Pat. No. 4,096,315 (NASA) describes a method for coating an optical plastic substrate with an abrasion resistant coating, deposited by a low temperature plasma polymerization process. Again, there is no mention of securing a uniform spread over the surface of the substrate. No discolouration is claimed to be witnessed when stored at temperatures of 170° F. (76.67° C.) for 168 hours. However it is to be noted that this method focuses on coatings for optical plastic substrates, in particular lenses for cameras, projectors, telescopes and other optical instruments, not for darker substrates, hence a transparent coating is needed on the substrates of U.S. Pat. No. 4,096,315. The problem the current invention aims to solve, poses itself primarily on darker substrates and therefor the claim of the patent of NASA that there was no evidence of discolouration, is to be taken in perspective of this context. Furthermore, the U.S. Pat. No. 4,096,315 concerns thin coatings with a preferable thickness of 20 nm. The applicant has noticed that, the thicker the applied coating is, the more pronounced the discolouration becomes, thereby becoming a very real problem in the desired range of coating thickness of the applicant.
The present invention relates to reaction chambers, for example for plasma processing, having a larger volume so as to treat multiple samples in a single processing run. This allows to have a high throughput combined with excellent performance of the treatment. The present invention contributes to improvement of the process in terms of visual effect, while at the same time the other characteristics of the treatment, e.g. a plasma coating, are maintained.
Nowadays, one of the critical parameters in adopting a technology is the throughput, the number of pieces that can be treated in one day, one week, one month or one year. To answer the increased throughput requests of the customers in different markets, larger systems have been constructed. Small R&D systems of less than 1 litre of useful area have been scaled up by extensive research over the years, to systems of several hundreds to thousands of litres. For example, the applicant has developed batch systems having a volume of 500 litres—wherein for example up to 300 smartphones can be treated in a single batch—and even larger, up to 10 000 litres for roll-to-roll systems used for coating rolls of textile.
The main challenge for these large production scale machines is how the plasma density or the plasma intensity can be distributed evenly and homogeneously over the complete useful area. It is well known that the plasma uniformity and plasma distribution of such large systems is less that in small R&D systems. Research has been done to optimize the uniformity, but it is very difficult to even impossible to have a 100% uniform plasma distribution because of the fact that many components of the equipment are implemented in the chamber: pump openings, gas inlets, trays/hangers, electrodes, etc.
Consequently, discolouration on substrates which are dark and/or are having a high gloss and/or a smooth surface, did show up due to uneven plasma uniformity inside the chamber. Optimization of the plasma parameters did not provide solutions for this discolouration issue.
Discolouration limits the applications and markets where the plasma process may be used for added value to products. The use of plasma coatings is limited especially for use on finished good level since these products are meant for direct sales to customers. Of course customers are not likely to buy items that have discolouration, a non-homogeneous colour or a rainbow-like shine.
Examples of finished goods where the use of plasma coatings might be limited due to discolouration issues are hand held electronic devices in the retail market, e.g. smartphones, mobile phones, tablets, personal digital assistants (PDAs), navigation systems, speakers, heading airs, headsets, and so on. Other examples are textiles for garments and clothing, such as sports and outdoor clothing, shoes and equipment, or shoes, clothing and equipment for use in personal protective equipment (PPE)—medical applications, cleanroom, firemen, policemen, postmen, etc.